For many years there has been recognition that vehicles could be made more fuel-efficient if the energy normally lost in decelerating or braking the vehicle could be somehow collected, stored and reused to accelerate the vehicle. A relatively large number of prior patents and published patent applications exist which are directed to various aspects of this general approach. Some have proposed to collect and store the energy in hydraulic accumulators and then reuse the energy through fixed or variable displacement hydraulic transmissions. A variation of this concept utilized a flywheel as an energy storage device for collecting and storing vehicle deceleration energy, either alone or in combination with a hydraulic accumulator.
Many of the previously disclosed systems do not lend themselves to use in existing truck designs whereas others require specially fabricated hydrostatic drive components.
Some systems heretofore have used a transmission including a hydrostatic transmission portion and a mechanical transmission portion. A typical hydrostatic transmission comprises a variable-displacement pump hydraulically coupled to a motor (typically, of fixed displacement), and appropriate controls for varying the displacement of the pump. The “mechanical transmission” may comprise a two-speed, shiftable, gear-type transmission that allows the hydrostatic transmission to use smaller and less expensive components. A problem with some hydrostatic transmissions has been the need to stop the vehicle to shift between different gear ratios.
One solution to this problem is described in U.S. Pat. No. 6,202,016 which discloses a hydrostatic transmission that can be shifted “on the go”. The transmission includes a mechanical transmission that is provided with a shift cylinder having a neutral position, a low gear position and a high gear position. The hydrostatic transmission is electronically controlled so that displacement of a variable displacement pump is coordinated with the shifting of the mechanical transmission to achieve the shift-on-the-go capability. The transmission disclosed in this patent, however, limits the ability to optimally control engine speed and pump displacement independently of existing driving conditions, nor does such transmission lend itself to efficient operation at “on highway” speeds, typically speeds greater than 40 mph.